1. Field of the Invention
The present invention relates to a CDMA (Code Division Multiple Access) receiving apparatus for a mobile communication system, particularly for a car phone/portable phone system (cellular system), and a reception method therefor.
2. Description of the Related Art
The CDMA reception method has an advantage that the same frequency is usable between all cells and within a cell, and a path diversity effect of composing plural arrival path signals can be obtained by a simple RAKE receiver. However, it has a problem that the interference between channels cannot be perfectly extinguished. In order to solve this problem, Japanese Laid-open Patent Application No. Hei-7-170242 has disclosed a method of removing interference by using an adaptive despreading filter.
FIG. 4 shows the construction of the conventional CDMA receiving apparatus disclosed in the above publication.
The conventional CDMA includes a reception antenna 101 for receiving a radio signal transmitted from a communication partner station (i.e., a station with which a station concerned communicates), a radio reception unit 102 for demodulating the radio signal received by the reception antenna 101 to convert the radio signal to a base band signal, an analog/digital converter (A/D) 103 for converting the base band signal to a digital signal and outputting it as a reception signal, a data accumulator 104 for accumulating the reception signal converted by the analog/digital converter 103, a channel signal processor 105 and a clock generator 106 for transferring a clock signal clk to each block of the channel signal processor 105.
In each block of the channel signal processor 105, a signal having a twice period of the chip rate serving as a reference time unit of the frequency spreading processing is formed by using the clock signal clk, and each block operates on the basis of the signal thus formed.
Further, the channel signal processor 105 comprises an adaptive despreading filter 110, an adaptive controller 111, a judgment unit 112 and an autocorrelative inverse matrix calculator 113.
The adaptive despreading filter 110 comprises a transversal filter, and it applies filtering processing to the reception signal r by performing despreading with a variable coefficient vector a.
The adaptive controller 111 renews the variable coefficient vector a so that the error power between the output signal re and the judgment data rd is minimum.
The judgment unit 112 makes a judgment on the output signal re of the adaptive despreading filter 110 so that the output signal re is set to the data of a transmission series of a transmitter which transmits the radio signal, and outputs the judgment result as the judgment data rd. That is, the judgment data rd is the data obtained by removing noises from the output signal data re.
The autocorrelative inverse matrix calculator 113 calculates the inverse matrix of an autocorrelative matrix of the reception signal re, and transfers the inverse matrix thus calculated to the adaptive controller 111.
FIG. 5 is a block diagram showing the adaptive despreading filter 110. The adaptive despreading filter 110 comprises sampling circuits 201, 205, a shift register 202, a variable coefficient multiplier 203 and an adder 204.
The sampling circuit 201 samples the reception signal r at the twice period of the chip rate, and outputs it to the shift register 202.
The shift register 202 successively stores the reception signal r sampled by the sampling circuit 201 to generate and output a tap output vector u.
The variable coefficient multiplier 203 multiplies the tap output vector u and the variable coefficient vector a with each other.
The adder 204 adds the multiplication results of the variable coefficient multiplier 203.
The sampling circuit 205 samples the addition value of the adder 204 at the period of a symbol rate, and outputs it as the output signal re.
Next, the operation of the conventional CDMA receiving apparatus thus constructed will be described hereunder.
First, the radio signal received by the reception antenna 101 is demodulated by the demodulator 102 to be converted to the base band signal, converted to the digital reception signal r in the analog/digital converter 103, and then accumulated in the data accumulator 104.
The reception signal r accumulated in the data accumulator 104 is sampled at the twice period of the chip rate in the sampling circuit 201, and then input to the shift register 202.
Representing the spreading rate of CDMA by M, for the shift register 202, the tap interval is equal to the half of the period of the chip rate, and the number of taps is equal to 6M.
In the autocorrelative inverse matrix calculator 113, the inverse matrix of the autocorrelative matrix of the reception signal r is calculated, and transferred to the adaptive controller 111. In the adaptive controller 111, the variable coefficient vector a={a0, a1, a2, . . . , a6Mxe2x88x921} serving as a filter coefficient of the adaptive despreading filter 110 is generated by using the inverse matrix thus calculated.
Thereafter, in the adaptive despreading filter 110, the reception signal r is sampled by the sampling circuit 201, and successively stored in the shift register 202 to generate the tap output vector u={u0,u1,u2, . . . , u6Mxe2x88x921} and output the tap output vector to the adaptive controller 111. Further, in the adaptive despreading filter 110, the tap output vector u is weighted with the variable coefficient vector a and added in the adder 204, then sampled at the period of the symbol rate in the sampling circuit 205, and then output as the output signal re. In the judgment unit 112, noises are removed from the output signal re, and the resultant signal is output as the judgment data rd.
Here, the adaptive controller 111 controls the variable coefficient vector a so that the error power between the output signal re and the judgment data rd is minimum. A known training signal is transmitted/received at a transmission side/reception side from the time just after reception of a burst signal is started until the variable coefficient vector a of the adaptive despreading filter 110 is converged. After the variable coefficient vector a is converged, correct data are output for the judgment data from the judgment unit 112, so that the variable coefficient vector a is successively judged so that the adaptive controller 111 minimizes the error power between the output signal re from the adaptive despreading filter 110 and the judgment data rd from the judgment unit 112, whereby the variation of the transmission path characteristics can be tracked. As an adaptive control algorithm for minimizing the error power have been known an LMS (Least Mean Square Algorithm) which is simple, but low in converging rate, an RLS algorithm which is high in converting rate, but complicate, etc.
Signal components (interference signals) addressed to other receivers and noises of the receiver cause errors in the self-receiving apparatus. Accordingly when power of interference signal noise power of receiver, the adaptive despreading filter 110 operates to cancel the interference signal.
In the conventional CDMA receiving apparatus, the composite path length, that is, the tap number of the adaptive despreading filter 110 is fixed to the maximum value in advance. Therefore, when a communication is made under a severe radio environment like a metropolitan region containing many buildings or the like in which multipath occur frequently because the radio transmission path status is bad, the multiple paths can be added without omission by composing many paths. Therefore, S/N is increased and an error rate is reduced, and thus this is extremely effective. However, for example when a communication is made under a radio environment like a clear suburb in which multipath occur rarely because the radio transmission path status is relatively good, the delay dispersion is smaller than a predetermined composite path length, and unneeded taps exist. Therefore, the adaptive despreading filter 110 operates these unneeded taps, and thus it performs unneeded calculation. Further, for the calculation in the adaptive controller 111 and the inverse matrix calculation in the autocorrelative inverse matrix calculator 113, unneeded calculations are carried out because of existence of unneeded taps.
In a portable telephone (cellular phone) system, it is a critical problem to reduce the power consumption of telephones owing to the recent miniaturization design of telephones, and the unneeded calculations promote extra power consumption.
Further, when the variable coefficient vector a corresponding to the unneeded tap is not zero, noises are unintentionally added, resulting in deterioration of S/N or increase of the error rate in some cases.
The above-described conventional CDMA receiving apparatus has the following problems.
(1) when a communication is made under such a radio environment that the radio transmission path state is relatively good and thus the multipath is less, unneeded taps exist, so that power is vainly wasted.
(2) When a communication is made under such a radio environment that the radio transmission path state is relatively good and the multipath is less, unneeded taps exist, so that noises are added to induce deterioration of S/N and increase of the error rate.
The present invention has an object to provide a CDMA receiving apparatus which can reduce calculation load to suppress power consumption and also enhance reception quality by operating an adaptive despreading filter by using only a needed number of taps.
In order to attain the above object, a CDMA receiving apparatus according to the present invention is characterized by comprising:
a reception antenna for receiving a radio signal transmitted from a communication partner station;
a radio reception unit for demodulating the radio signal received by the reception antenna to convert the radio signal to a base signal;
an analog/digital converter for converting the base band signal to a digital signal and outputting the digital signal as a reception signal;
an adaptive despreading filter for performing despreading with a variable coefficient vector to subject the reception signal to filtering processing;
an adaptive controller for renewing the variable coefficient vector so that the error power between the output signal of the adaptive despreading filter and the judgment data is minimum;
a judgment unit for making a judgment so the output signal from the adaptive despreading filter is data of a transmission series of a transmitter transmitting the radio signal, and outputting the judgment result as the judgment data;
an autocorrelative inverse matrix calculator for calculating the inverse matrix of an autocorrelative matrix of the reception signal and transferring the inverse matrix thus calculated to the adaptive controller;
a profile calculator for calculating profile data comprising the level of a signal by despreading the reception signal with a timely-shifted spread code, and the timely-shifted amount of the spread code; and
a tap controller for determining the needed tap length and the timing from the profile data.
According to the present invention, the profile data of the reception signal are created in the profile calculator, and the number of needed taps and the timing are determined by using the profile data in the tap controller, whereby the calculation for only needed number of taps are performed in the adaptive controller and the autocorrelative inverse matrix calculator and no calculation is performed for unneeded taps in the adaptive despreading filter.
Accordingly, the calculation load is reduced and thus the power consumption is reduced, so that any noise due to unneeded taps does not occur, thereby enhancing the reception quality.